In the inventor's U.S. Pat. No. 3,710,340 there is described a quasi-planar antenna consisting of two resonant elements of the dual type fed simultaneously from the same transmission line. The resonant elements are adjusted so that the image impedance of the composite two-port structure is nearly constant and resistive over a very wide frequency band. In a given plane, one element (i.e., a monopole) has an omnidirectional pattern and the other (i.e., a slot), a typical figure eight dipole pattern. Proper adjustment of feeder impedance relative to the impedances of the individual elements leads to a undirectional pattern with high front-to-back ratio over the operating band width.
To achieve directionality of transmission in the inventor's previous structure, required that a cavity be emplaced around a slot cut in the ground plane. While operational, this provided a rather unwieldy structure having substantial protrusions on both sides of the ground plane.
Other attempts at constructing planar microwave antennas are illustrated in U.S. Pat. Nos. 3,665,480 and 4,208,660. Both of these patents describe a system wherein an annular opening in a ground plane is fed via a stripline arrangement in such a manner that the propagation pattern from the antenna is orthogonal to the planar dimension of the antenna. In the '480 patent, the frequency of transmission is directly related to the dimension of the annular opening, whereas in the '660 patent a plurality of annular openings are provided so as to enable a multifrequency transmission. Neither of these structures provide a planar antenna wherein the pattern maximum is in the plane of the antenna and is selectable in azimuth.
Accordingly, it is an object of this invention to provide a substantially planar antenna whose direction of propagation is parallel to and either omni- or multi-directional over the plane of the antenna.
It is another object of this invention to provide a substantially planar antenna whose input impedance is substantially constant and resistive over a wide band of frequencies.
It is a further object of this invention to provide a substantially planar antenna, which in the receiving mode responds to both electric and magnetic fields.